1. Field of the Invention
This invention relates to thermal imager testing devices.
2. Description of Related Art
Thermal imager testing devices are used in the testing of thermal imagers so as to avoid the necessity of field trials for the imagers. This application requires that the device present to the thermal imager pictures sufficiently similar to those actually seen in the field for the test to be realistic.
One example of a known thermal imager testing device uses a vanadium dioxide thin film having a thermodynamic phase transition at a predetermined temperature which causes the film to switch from having high infrared transmission to low infrared transmission. The existence of a hysteresis in this phase transition enables the film to be used as an optical storage medium, such that by heating portions of the film above the transition temperature, and placing the film in an optical projector operating in the infrared, scene information written on the film may be projected into the field of view of a thermal imager to be tested.
Such a device suffers the disadvantages however of lack of accuracy and the inability to incorporate a grey scale.
Another example of a known thermal imager testing device comprises a nematic liquid crystal cell incorporating a photoconductive layer. An image shone on the photoconductive layer causes the molecules within selected areas of the liquid crystal to change their orientation within the cell, thus changing the transmission characteristics of incident infrared light through the cell.
Such a device suffers the disadvantage however that the response time of the liquid crystal to changes in the image is relatively slow.